Heat transfer occurs in many technical processes, for example in refrigeration and air conditioning technology or in chemical and energy technology. In heat exchangers, heat is transferred from one medium to another medium. The media are usually separated by a wall. This wall serves as a heat transfer surface and for separating the media.
In order to enable the transporting of heat between the two media, the temperature of the heat-releasing medium heat has to be higher than the temperature of the heat-absorbing medium. This temperature difference is referred to as the driving temperature difference. The higher the driving temperature difference is, the more heat that can be transferred per unit of the heat transfer surface. On the other hand, attempts are frequently made to minimize the driving temperature difference since this has advantages for the efficiency of the process.
It is known that heat transfer can be improved by the structuring of the heat transfer surface. In this way, the effect can be achieved of more heat being able to be transferred per unit of the heat transfer surface than in the case of a smooth surface. Furthermore, it is possible to reduce the driving temperature difference and therefore to make the process more efficient.
Shell and tube heat exchangers are a frequently used design of heat exchangers. In these devices, use is frequently made of tubes which are structured both on their inner and outer sides. Structured heat exchanger tubes for shell and tube heat exchangers usually have at least one structured section and also have smooth end pieces and possibly smooth intermediate pieces. The smooth end pieces or intermediate pieces delimit the structured sections. So that the tube can be installed in the shell and tube heat exchanger without any problem, the outside diameter of the structured sections must not be larger than the outside diameter of the smooth end pieces and intermediate pieces.
Various measures are known for increasing heat transfer during the condensation process on the tube outer side. Fins are frequently applied to the outer surface of the tube. As a result, the surface of the tube is primarily increased and consequently the condensation process is intensified. For heat transfer, it is especially advantageous if the fins are formed from the wall material of the smooth tube since an optimum contact between fin and tube wall then exists. Finned tubes, in which the fins have been formed by means of a forming process from the wall material of a smooth tube, are referred to as integrally rolled finned tubes.
Today, commercially available finned tubes for condensers have a fin structure on the tube outer side with a fin density of 30 to 45 fins per inch. This corresponds to a fin pitch of approximately 0.85 to 0.55 mm. A further performance enhancement by increasing the fin density is limited by the inundation effect which occurs in shell and tube heat exchangers. With the spacing of the fins becoming smaller, the interspace of the fins is flooded with condensate as a result of the capillary effect and draining off of the condensate is hindered as a result of the channels between the fins being made smaller.
The prior art further increases the surface of the tube by introducing notches in the fin tips. Additional structures, which positively influence the condensation process, are also created as a result of the notches. Examples of notches in the fin tips are known from printed documents U.S. Pat. No. 3,326,283 and U.S. Pat. No. 4,660,630.
Furthermore, it is known that performance enhancements can be achieved in condenser tubes by additional structural elements being introduced between the fins in the region of the fin flanks with a constant fin density. Such structures can be formed on the fin flanks by means of toothed wheel-like tools. The material projections which are created in this case project into the interspace of adjacent fins. Embodiments of such structures are found in printed documents DE 4404357 C2, CN 101004335 A, US 2007/0131396 A1 and US 2008/0196876 A1. The material projections which are described in these printed documents extend in the axial and circumference directions of the tube. In US 2010/0288480 A1, it is proposed to form the material projections so that they are delimited by one or more convexly curved surfaces. In printed documents CN 101004337 A and US 2009/0260792 A1, additional material projections, which extend in the main in the axial and radial directions, are shown on the fin flank. These material projections are arranged in the circumferential direction on the edges of the material projections and formed approximately perpendicularly to these. Consequently, each radially extending material projection has a common boundary line with a circumferentially extending material projection. Along this boundary line, the axial extent of both material projections is equal. As a result, pocket-like structures, which are delimited in each case by three material projections and the fin flank, are created on the rib flank. The condensate accumulates preferably in these pocket-like structures on account of capillary forces. As a result, further condensation of vapor is hindered and the performance of the tube is reduced.